Although many different types of filtration are known, all such methods require energy to separate a filtrate from a retentate. Energy can be provided in many different forms, including centrifugal force, gravity, pump pressure, capillary action, etc.
The parameters that determine the energy demand of filtration vary among the different types of filtration. In reverse osmosis systems, for example, energy demands are proportional to the size of the particles, the concentration of the particles, and the osmotic pressure of the particles. Reverse osmosis is a process in which ions or small molecules are removed from a feed fluid by forcing the feed fluid under high pressure through a semipermeable membrane. Examples of reverse osmosis can be found in U.S. Pat. No. 5,500,113 to Hartley et al., U.S. Pat. No. 4,632,754 to Wood, and U.S. Pat. No. 5,853,599 to Hsu.
In many cases considerable amounts of energy must be spent to produce a filtrate from a feed fluid. The energy is typically provided by a pump. In the case of reverse osmosis, for example, pumps typically raise a fluid to an elevated reservoir, and the height difference between the reservoir and the membranes provides a head pressure used to force the filtrate through the membranes. Unfortunately, only about 20-33% of the fluid pumped under high pressure is filtrate, which means that much more fluid is pumped than is eventually produced as filtrate. This reduces the energy efficiency of the process.
In an alternative method of creating head pressure, a reverse osmosis system is placed deep in a fluid filled channel. The high pressure side of the semipermeable membrane contacts the fluid at a relatively high pressure corresponding to the vertical distance between the membrane and the reservoir. The low pressure side of the membrane contacts the filtrate column at a relatively low pressure corresponding to the vertical distance between the membrane and the pump. The pressure difference between the two sides of the membrane drives the reverse osmosis. A pump is needed to continually remove the filtrate in order to maintain the pressure difference.
Deep well based reverse osmosis systems provide for potentially higher energy efficiency than reservoir type systems because almost all of the fluid that requires high pressure pumping is filtrate. However, in order to recover the filtrate, the filtrate must still be pumped over a significant height (pressure) differential, which typically requires the operation of one or more submerged pumps.
Unfortunately, submerged pumps pose serious difficulties, especially when disposed at depths of several hundred meters. For example, power supply and additional wiring to control the operation of the pump must be run to each submerged pump. Casings for submerged pumps must be waterproof, which may be especially difficult when the pumps operate at depths below several hundred meters. A further problem is that when a submerged pump needs maintenance it must generally be retrieved from its operating position, and in most cases such retrieval halts operation of the system. The problem is further compounded by the need tightly mount the pump to the walls of the channel to prevent dislocation and vibration damage.
Air-lift pumps can be used to circumvent some of the problems associated with submerged pumps. The basic principle is that gas introduced at a depth has a natural tendency to rise upwards, and that the rising force tends to lift surrounding fluid upwards as well. A co-owned patent application, Ser. No. 09/014,238 filed Jan. 27, 1998, incorporated herein by reference in its entirety, describes embodiments in which an air-lift pump can be operated in conjunction with a pneumatically powered centrifugal pump to lift filtrate from a well-based reverse osmosis system. Using an air-lift pump is desirable because the introduction of air into a fluid can be performed at various depths with only minimal equipment. Another advantage of air-lift pumps is that they may require only a fraction of the space required for a mechanical pump. Still another advantage is that air-lift pumps are relatively maintenance free, having few or no moving parts.
Despite the many advantages of air-lift pumps, there are several problems. For example, in known air-lift pumps, a compressor is generally required to provide compressed air. Compressors may be relatively inefficient, and may require significant maintenance due to the use of moving parts. Moreover, the compressed air must be delivered to the point of introduction through high-pressure lines. Such lines are usually fabricated in relatively short lengths, and must be serially connected to achieve sufficient length in an underground channel. The connections and prone to leaks
It is possible to generate a gas in a fluid via a chemical reaction, but that strategy merely introduces other problems. The chemicals used for gas production would usually interfere with the purity of the filtrate, or at the very least tend to cause an undesirable change in the pH or other physicochemical parameters of the filtrate. Furthermore, the gas producing reaction must be initiated and run from a remote location, which may likely allow only minimal control over the progress of the reaction.
Another possibility is electrolysis. Although it is unknown to use electrolysis to produce gas for use in an air-lift pump, it is known to conduct electrolysis at a great depth. For example, in the French patent 2,298,613 to Imbreteche, seawater is electrolyzed at a considerable depth in a submerged device to produce pressurized oxygen gas and hydrogen gas.
In summary, many methods are known to recover a filtrate using reverse osmosis. However, current recovery systems tend to require complex equipment that is relatively inefficient or needs frequent maintenance. Surprisingly, despite numerous recovery systems for filtrate in reverse osmosis in a deep channel, there is no system that allows relatively simple and efficient recovery of filtrate from reverse osmosis. Therefore, there is still a need for methods and apparatus that resolve these problems.